Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this disclosure and are not admitted to be prior art by inclusion in this section.
RX supervision in a radio unit (including base station, user equipment and so on) is an important feature to supervise the sanity of the RX chain for radio characteristic and robusness, and to report an abnormal situation. A normal RX operation may be impacted by components quality, aging or broken by internal or external high power level interferes by wrong operation which could exceed prediction level in practical applications.
Cellular technologies specified by the 3rd Generation Partnership Program (3GPP) are the most widely deployed in the world. A new step being studied and developed in 3GPP is an evolution of 3G into an evolved radio access technology referred to as Long-Term Evolution(LTE). In LTE, different modes of communications can be used for radio nodes in a cellular network, such as Frequency Division Duplex (FDD), Time Division Duplex (TDD) and half duplex. In TDD radio, the uplink and downlink communication between a base station and a mobile phone uses the same frequency band (i.e., carrier) but different timeslots to separate the receive (RX) operation and transmit (TX) operation, i.e., the RX operation and TX operation take place in different, non-overlapping timeslots.
FIG. 1 shows a typical TDD radio unit. As shown, a typical TDD radio unit comprises a baseband processing circuit, a TX path, a Transmitter Observation Receiver (TOR) module, a circulator, a filter unit (FU), an antenna, a Transmit/Receive (TR) swich and an RX path. The baseband processing circuit processes a baseband signal and transmits it to the TX path for transmission over the antenna, and processes a baseband signal received from the RX path. The TX path converts the baseband signal to an RF signal so as to transmit it over the antenna. The RX path converts an RF signal received from the antenna to a baseband signal that will be processed by the baseband processing circuit. The TOR module is coupled to receive an output of the TX path and provide feedback signals to the baseband processing circuit so as to perform calibration and pre-distortion measurements on the TX path. The circulator plays the role to separate the TX path and the RX path signal. Since the RX operation and the TX operation of the TDD radio unit use the same frequency band in a TDD system, a single two-port cavity filter unit may be shared for both the RX path and TX path of the radio unit. During the TX operation of the radio unit, the TR switch is switched to its termination, as shown in FIG. 1. A reflected TX signal from the antenna or the filter unit will go through the TR switch and absorbed by the termination of the TR switch. Generally, the terminaltion is a 50 ohm high power termination. During the RX operation of the radio unit, the TR switch is connected to the RX path. The normal RX operation may be impacted by components quality, aging and other factors. For illustration, the TX path comprises a TX low level block, a power amplifier (PA), and a TOR Local Oscillator (LO). The RX path comprises an RF Low Noise Amplifier (LNA) block, an RX Local Oscillator (LO), and an RX block. The components of the TX path are set in a power-on state for a TX operation, while the components of the RX path are set in a power-on state for an RX operation.
The RX path of a TDD radio unit is generally designed under certain input power level limitation and could be broken if an input signal exceeds a prediction level. In a TDD system there are potential risks for the RX path, which are listed below. FIG. 2 shows the potential risks.    1. The RX front-end (i.e., the LNA block) could be possibly broken by a high power level due to misaligned TDD switching from its self TX operation by a wrong operation when there is no hardware fault prevention of TDD control. It is shown by a dashed line in FIG. 2, and may be classified as an internal interference.    2. RX/TX un-alignment due to a timing error or no co-operation among operators may cause a TX signal of a TDD radio unit to damage the RX front-end of other radio units in the same sector, which may cause a risk to damage the RX path of the other radio units. This scenario may also happen for example when co-sitting between a TDD system and a WiMax system. It is shown by a dotted line in FIG. 2, and may be classified as an external interference.
FIG. 3 shows a typical RX protection method used in a TDD radio unit, where an RF limiter is placed in front of the LNA block of the RX path. When a high level interfering signal enters the RX path, the RF limiter limits the power of the input signal to a certain safe zone for the subsequent LNA block.
However, this solution is only targeting to radio hardware protection against interference from outside. It will not guarantee an even higher input power level. The leakage power after the RF limiter will possibly saturate every component after the LNA block if the RX path is still working.
Above all, in TDD radio unit, the RX path is more fragile. So, there is a need to have an RX supervision function, reporting a status concerning the sanity of the gain of the RX path.
There have been proposed some RX supervision technologies. Some of the RX supervision technologies that are used in product comprise the following:    1. LNA current monitoring. This solution will monitor the current variation and report an error or warning by a multi-channel ADC when the supervised current variation value is out of limit.    2. Test tone injection. This solution will inject a test tone generated by a synthesizer whose frequency is located in the middle of the guard band of the radio unit normally. Together with the LNA current monitoring, the entire RX path will be supervised.
Still there are some RX supervision technologies which, among other, comprise the following:    3. Usage of an in-band noise floor measurement in RX digital part to test the sanity of the RX path from the LNA block to the end of the RX path via an in-band power detector interface.
The above existing technologies of RX supervision have respective problems.    1. For LNA current monitoring, the drawbacks are:            LNA current supervision cannot supervise the entire RX path.        Current cannot directly reflect the radio performance.            2. For test tone injection, the drawbacks are:            Test LO leakage could aggravate the clock leakage on board.        LO injection must be behind of the LNA block to avoid introducing extra RX noise figure, which means that LO injection can't provide an entire RX supervision.        Test tone injection needs an extra hardware of the synthesizer, and thus increases the PCB size.            3. For in-band noise floor measurement, the drawbacks are:            The noise floor estimation relies on an assumption that during sufficiently long observation time (hours) and regardless of a particular air interface, a measured carrier power will be at the noise floor level, i.e., no UE signal and/or interference will be present for a certain long time.        The assumption causes the method to be almost impractical.        
Therefore, there is a need for an improved RX supervision method.